Various types of leather-like sheets having a flexibility resembling natural leathers have been known. For example, a leather-like sheet which is produced by impregnating a polyurethane resin into an entangled nonwoven fabric made of microfine fibers of 1 dtex or less and laminating a film formed by coating a polyurethane resin on a release paper to a substrate obtained by wet-coagulating the impregnated polyurethane resin; a leather-like sheet which is produced by applying a polyurethane solution on a substrate obtained in the same manner as above, wet-coagulating the impregnated polyurethane solution, and then gravure-coating a colorant for the polyurethane resin; and a leather-like sheet which is produced by impregnating a polyurethane resin into an entangled nonwoven fabric made of sea-island fibers, wet-coagulating the impregnated polyurethane resin, converting the sea-island fibers to bundles of microfine fibers with a fineness of 0.2 de or less by removing one component from the sea-island fibers by resolution in a solvent thereby to obtain a substrate made of the bundles of microfine fibers, and then finishing the surface have been proposed (Patent Document 1). However, these leather-like sheets have a strong rubbery compression resistance which is characteristic of polyurethane resins. Thus, a leather-like sheet combining a low compression resistance and a dense feel each resembling natural leathers and having an appearance of fine bent wrinkles and a sufficient practical strength has not yet been obtained (Patent Documents 2 to 4).
The above leather-like sheets are produced by a method using a large amount of organic solvents. Such a method is complicated to increase the production costs and the lead time is necessarily prolonged. In the surface formation (formation of a grain surface layer) by a release paper method or a gravure coating method, a water dispersion of an elastic polymer may be used. However, the elastic polymer in the water dispersion is less compatible with the elastic polymer in the entangled nonwoven fabric. In addition, the grain surface layer is likely to be peeled off along the interface between the grain surface layer and the entangled nonwoven fabric because the water-dispersed elastic polymer is less cohesive, thereby failing to show a sufficient surface strength. If a production line using organic solvent widely employed is used as a production line using a water-dispersed elastic polymer, volatile organic compounds (VOC) are emitted. To achieve a production method with a low VOC emission and a small environmental load, a new line must be constructed to increase the initial investment. Therefore, an environmentally friend, rational production method of grain-finished artificial leathers have been demanded. However, a production method which meets the demand has not yet been proposed.
Artificial leathers composed of a fibrous substrate and an elastic polymer have been widely used as a substitute for natural leathers in the production of sheet materials for interior, upper material for shoes, sub-material for shoes, and material for clothes and pouches. Of suede-finished artificial leathers, nubuck-finished artificial leathers and grain-finished artificial leathers, the grain-finished artificial leathers are widely used for the application of shoes, balls, clothes, pouches, and interior. To make a grain-finished artificial leather aesthetically appealing, the color and pattern of its surface are made imitative of those of natural leather surface by a surface finishing according to its application. A surface finish resembling so-called pull-up natural leathers, in which the pull-up oil added to leather moves upon bending to generate natural unevenness because of the change of color shade at the bending portion, have been attempted in various applications. However, it was found to be not practical for known leather-like sheet because the surface strength was poor. In view of the environmental protection, an environmentally friend method has been recently required also in the production of leather-like sheets. However, in the known production methods of leather-like sheets, the use of organic solvents is needed to dissolve resins. The organic solvents may injure worker's health and the emitted organic solvents may cause atmospheric pollution.
To enhance the aesthetical appeal of a grain-finished artificial leather, Patent Document 4 proposes the use of a surface coating agent mainly composed of a polyurethane resin and blended with polybutylene and silica. Patent Document 5 proposes to impregnate an oil-soluble surfactant into an artificial leather. However, the proposed methods cannot reproduce the natural, massive, oil-up feeling of natural leathers.
Patent Document 6 proposes to coat artificial leather with wax to improve the color fastness of suede-finished artificial leathers. It is described that a nap-finished sheet with a good color fastness is obtained by coating a napped surface of microfine fibers with wax, heat-treating to raise the napped fibers laid down by wax, and then brushing. Thus, the technique proposed by Patent Document 6 is irrelevant to the oil-up effect.
Patent Document 7 describes that the color lightness reversibly changes upon bending when the open pores in a porous polyurethane layer of a grain-finished artificial leather is filled up with a wax with a melting point of 40 to 100° C. However, the open pores in the porous polyurethane layer are formed by a mechanical abrasion and a wax solution in an organic solvent should be used to fill up the open pores with wax. Therefore, the propose method should use, in addition to wax, a harmful organic solvent and include complicated production steps.
Patent Document 8 proposes a leather-like sheet which is covered with a nap of colored microfine fibers with a fineness of 0.1 dtex or less and a polymer which is solid at ordinary temperatures and has a melting point of 60° C. or higher and an elongation at break of 10% or less. It is taught that a color shade is obtained according to the degree of separation between the polymer and the microfine fibers and the degree of cracking of the polymer. However, the polymer which is solid at ordinary temperatures and brittle inevitably falls off from the surface layer, and therefore, the proposed leather-like sheet cannot withstand a long-term use.
Patent Document 9 describes a leather-like sheet which is composed of a substrate made of a fiber assembly and a polymer coating layer, a colorant-containing polyurethane elastomer layer (I) on the surface of the substrate, and a colorant-containing polyurethane elastomer layer (II) on the polyurethane elastomer layer (I). It is described that a massive color change is obtained by partly abrading the polyurethane elastomer layer (II) to expose the polyurethane elastomer layer (I). However, as compared with the color shade of natural leathers, the obtained color change is still artificial and the aesthetic appearance resembling natural leathers is not obtained.
As mentioned above, artificial leathers have found a wide application such as sport shoes, clothes, and gloves because of their softness, high quality and easy care. In view of recently increasing requirements of customers to the variety of feeling and performance, it is required to provide leather goods having a feeling and performance ever known. For example, during the use of sport shoes and gloves, the wearer's sweat and the increase in the inner temperature cause an unpleasant damp and hot feeling on feet and hands. To reduce the unpleasant damp and hot feeling under wearing, various artificial leathers have been proposed (Patent Documents 10 and 11). However no satisfactory artificial leather in practical use is obtained.
Various leather-like sheets have been proposed as the substitutes for natural leathers. Materials for grips of golf clubs and tennis rackets, materials for game balls, and materials for heals and soles of shoes are required to be well grippable in both the conditions where the surface is dry or the surface is wet by sweat or rain. For example, a number of pebbles (embosses) of a size of about 3.0 mm2 are provided on the surface of basket balls. However, since the handling ability and grippability during play are not sufficiently improved by only forming pebbles, it has been widely employed to coat the surface with a resin to improve the handling ability and grippability. However, the grippability in wet condition is not improved by only coating a resin, and the grippability is remarkably reduced by sweat during play. To improve the grippability in wet condition, there have been made many proposals to provide microholes for absorbing water and sweat on the upper or side surface of pebbles formed on the surface of materials.
Patent Document 12 describes to form a pebble-valley pattern on the surface by embossing and form microholes on the pebbles by buffing with sandpaper or a card clothing or by dissolving the surface with a solvent. Patent Document 13 describes a leather-like sheet which is produced by applying an elastic polymer on the surface of a substrate composed of microfine fibers and an elastic polymer, forming a pebble-valley pattern on the surface by an emboss roll, and then, forming a coating layer of an elastic polymer on the top surface of pebbles. The side wall between the top surface of pebble and the surface of valley has through holes which extend from the surface layer to the substrate layer. It is described that the through holes are formed by extending the side wall of pebbles during the emboss treatment.
However, the proposed leather-like sheet is still insufficient in the wet grippability. In addition, since the difference between the dry grippability and the wet grippability is large, the handling ability remarkably changes during play. Further, additional production steps are necessary to form the microholes and through holes. Therefore, it is required to improve the production efficiency.
Artificial leather strings obtained by cutting a leather-like sheet having a softness resembling natural leathers have been used in the production of clothes and woven or knitted fabrics for interior goods, or used as laces for shoes, bags and baseball gloves or braids for fancyworks. However, the artificial leather strings obtained by cutting a known leather-like sheet have a poor strength. Thus, an artificial leather string having a strength comparable to the strings obtained by cutting natural leathers have not yet been obtained.
Patent Document 14 discloses a leather string-like yarn composed of a fibrous substrate having a grain surface on its one surface, which has different colors on its two surfaces. It is described that the leather string-like yarn has excellent mechanical properties such as a high strength, an improved elasticity and an improved firmness. However, there is no objective evidence to show such excellent mechanical properties.
Natural leathers produce fine wrinkles in every direction on its surface with use for a long time and show antique appearance. Natural leather products having an antique appearance and an attractive vintage feel are accepted by many users as high quality fancy goods. Therefore, it has been required in the field of artificial leathers to develop a leather-like sheet which can form an antique appearance resembling natural leathers. To meet the requirement, various semigrain-finished leather-like sheets have been proposed. The proposed semigrain-finished leather-like sheets are produced by a production method including a step of raising the surface of a fibrous substrate by buffing and a step of applying the napped surface with an elastic polymer to control the length of napped fibers. However, the semigrain-finished leather-like sheet produced by such a method has a hard, rubbery and plastic surface because the surface is covered with a continuous film of the elastic polymer. Therefore, such a semigrain-finished leather-like sheet forms only wrinkles which are seen artificial at a glance even after a long term use, and a well-worn antique appearance resembling natural leathers is not obtained.
Patent Document 15 discloses a leather-like sheet having a coating layer with a micro joint structure on at least one surface of the substrate. The coating layer with a micro joint structure is formed by mechanically and/or chemically finely dividing a continuous film formed on at least one surface of the substrate. It is described that the micro joint structure provides an extremely natural appearance not ever obtained. However, it is still difficult to form an antique appearance resembling natural leathers on the surface of the proposed leather-like sheet.
The known leather-like sheets are all produced by methods using many organic solvents. In addition, the known methods include complicated production steps, this increasing production costs and necessarily resulting in a long lead time. In the surface forming step (step of forming a grain surface layer) by a release paper method or a gravure coating method, a water dispersion of an elastic polymer is usable. However, the elastic polymer for the grain surface is less compatible with the elastic polymer in an entangled nonwoven fabric. In addition, since the water-dispersed elastic polymer is less cohesive, the grain surface is easy to peel off from the entangled nonwoven fabric at their boundary, thereby failing to obtain a sufficient surface strength. If a production line using organic solvent usually employed is used as a production line using a water-dispersed elastic polymer, volatile organic compounds (VOC) are emitted. To achieve a production method with a low VOC emission and a small environmental load, a new line must be constructed to increase the initial investment. Therefore, an environmentally friend, rational production method of semi grain-finished artificial leathers have been demanded. However, a production method which meets the demand has not yet been proposed.    Patent Document 1: JP 63-5518B    Patent Document 2: JP 4-185777A    Patent Document 3: Japanese Patent 3187357    Patent Document 4: JP 61-285268A    Patent Document 5: JP 1-139877A    Patent Document 6: JP 3-25551B    Patent Document 7: Japanese Patent 3046174    Patent Document 8: JP 2002-30580A    Patent Document 9: JP 1-266283A    Patent Document 10: JP 8-41786A    Patent Document 11: JP 9-59882A    Patent Document 12: JP 2004-300656A    Patent Document 13: JP 2006-89863A    Patent Document 14: JP 59-150133A    Patent Document 15: JP 9-188975A